1. Field of the Invention
This invention relates generally to dissolved air floatation (DAF) systems, and more particularly to a new improved method and apparatus for DAF systems and other gas-liquid contacting systems for separating and reacting liquids and contacting gases.
2. Description of the Prior Art
Many industrial activities are sources of gases, liquids, and gas and liquid wastes containing condensable organic vapors and emulsified or nonemulsified suspended matter such as oils, fats, greases, metals, organic and inorganic solids which must be separated or treated before they can be used in any subsequent process or before they are discharged into the environment. These activities include petroleum and vegetable oil refining, steel making, food and beverage processing, metal working, chemical and paint manufacturing, pulp, paper and textile production, aircraft maintenance, coke-oven gas treatment, and water conditioning and treatment. The concentration of suspended matter and other constituents varies widely depending on the source and process conditions.
Treatment of the above liquids and liquid wastes is usually carried out in at least two stages. First, easily settled solids are separated from the liquid medium by gravity and floating oily constituents are skimmed off. Then stable emulsified liquids are broken down for separating the remaining oily constituents from water. Any remaining dissolved matter may be removed, if necessary, by one or more additional stages including extraction, adsorption, absorption, distillation, crystallization, membrane separation, and chemical separation.
Currently, three methods widely used for separating suspended matter from a liquid are gravity separation or sedimentation for heavier-than-liquid solids, filtration for small particles, and air floatation for floatable suspended matter. Of these, air floatation is the most versatile and widely practiced in the industry. Air may be bubbled through the liquid causing suspended matter to float to the surface where it can be skimmed off. However, very small suspended matter is not effectively separated by this method. A more effective method is by dissolved air floatation (DAF). Air at high pressure is dissolved in the liquid to be treated and introduced into a floatation tank at a lower pressure. Microbubbles of air 10 to 40 micrometers in size are released and rise gently through the liquid lifting floatable suspended matter to the surface.
A conventional DAF system feeds both the raw, untreated liquid and a liquid with dissolved air into the floatation tank through separate inlets located near the bottom. Treated liquid, still containing suspended matter but in reduced concentrations is discharged near the bottom of the tank while floated suspended matter is skimmed off and removed at the top. Since floatation of the suspended matter by rising air bubbles takes place vertically, there is a vertical concentration gradient of the suspended matter in the liquid in the floatation zone. Due to the buoyant force of the rising air bubbles, the untreated liquid in contact with the bubbles moves upward while the treated liquid moves downward. The vertical countercurrent flow of the two liquids produces turbulence and mixing of the two liquids and thereby reduces separation efficiency.
The turbulence and mixing may be somewhat minimized by reducing the inflow rate and by adding a baffle to direct the floc-bubble agglomerates toward the surface. The baffle, generally cylindrical for a cylindrical tank and flat for a rectangular tank, is placed approximately 60.degree. from the horizontal plane to reduce the velocity and the coalescence of the rising air bubbles. The liquid rising under the floated matter must turn at the top of the baffle approximately 180.degree. and flow downward to a discharge at the bottom of the tank. The turbulence created by the rising air bubbles and the floated matter flowing over the baffle causes mixing near the liquid surface. Consequently, the baffles are relatively ineffective. Baffles also drastically reduce the available surface area of the tank for floatation, and require a relatively tall tank to allow sufficient space above the baffles for floated matter to flow over the surface. Unpreventable turbulence in the contacting zone inside a baffle can also cause poor attachment of air bubbles to particles of the suspended matter and break-up of floc-bubble agglomerates.
It is generally impractical to separate very dilute concentrations (less than 10 ppm) or very small (less than 10 microns) suspended particles from a liquid medium by conventional DAF systems. Chemicals, such as polyvalent electrolytes and long-chain polymers, are used to coagulate and flocculate small particles to form larger aggregates which can be more readily contacted and attached by the air bubbles. The larger aggregates, being more buoyant and less likely to break up in turbulence, makes floatation easier and separation efficiencies higher. Efficiencies may range from 10 percent to 40 percent without the chemicals, and between 40 percent and 90 percent with chemicals. Paradoxically, the added chemicals may create more problems than they solve. They not only increase the cost of separation but also the process time. Coagulation can occur very rapidly in a fast-mixing tank, but flocculation takes time, normally about 20 to 30 minutes, and requires a slow-mixing tank. Only a very small fraction of the chemicals dissolved into the liquid are actually removed with the floated matter. The remaining chemicals may have to be removed later if their concentration is unacceptable for recycling or release to the environment.
A more recent DAF system for minimizing mixing between the untreated liquid and treated liquids divides the floatation tank into several communicating compartments and directs the liquid serially through each compartment. Separation is somewhat improved for large tanks, but only marginally for small tanks. For a given flow rate, mixing will be more pronounced with a limited number of small compartments.
Another system utilizes a long floatation tank with a liquid inlet at one end and a liquid outlet at the other end. Back-mixing of liquid may be reduced, but there are few DAF sites having sufficient space to accommodate such a long tank.
One of the most popular DAF systems recycles treated liquid back to the floatation tank, but there is a tradeoff. Throughput capacity is reduced, there is more back-mixing of the liquid being treated, and operating costs are higher. To compensate for these deficiencies, the system is often combined with other technologies, filtration being a frequent choice especially for producing potable drinking water. However, the combination of dissolved air floatation with such technologies invariably increases the complexity and the cost of the system.
Separation and removal of large, heavy solids which tend to settle at the bottom of a tank is another problem area. One solution uses a slow-speed agitator combined with a separate outlet for solids at the bottom of the tank, but undesirable liquid turbulence and mixing will occur.
From all of the above, it is apparent that turbulence and back-mixing of liquid inside conventional DAF tanks is a pervasive and continuing problem affecting separation efficiency. Baffles and low flow rates reduce the turbulence and back-mixing, but at the expense of throughput capacity. Chemicals facilitate the separation of liquids, but they also increase process time and costs, and manifest many undesirable side effects. Recycling treated liquid per se may improve separation efficiency but it also increases costs and back-mixing. Methods and apparatus for removing the floated matter and/or settled solids also contribute to the turbulence and back-mixing. None of these are capable of preventing turbulence and back-mixing of the liquid being treated without sacrificing throughput capacity. High separation efficiencies have been realized only with chemicals and time-consuming flocculation. Simultaneous removal of floated matter and settled solids may be achieved but with the undesirable effects of liquid turbulences.